#
# Calculate solar path in horizontal coordinates (altitude, azimuth)
# Use gnuplot's polar mode with inverted R range so that the horizon
# (altitude 0°) is at the perimeter and altitude 90° is at the center.
#
save_encoding = GPVAL_ENCODING
set encoding utf8
set label 1 at screen 0.02, 0.95 font ":Bold,10"
set label 1 "Solar path at\nLatitude 47.67 N"
set timestamp "Seattle - %d %b %Y" offset 1,1
# Set up equations for Azimuth(t) and Altitude(t) as a function of
# t = seconds before or after solar noon on the day of interest
# at a particular latitude.
# Here we calculate for Seattle (47.67 N -122.28 W)
 Latitude = 47.67
 Longitude = -122.28
 Date = "21-12-2016"
 call 'solar_params.dem'
 array dummy[150]
 array winter_alt[150]; array winter_azi[150]
 time = -12 * Hour
 do for [t = 1 : 150] {
	winter_alt[t] = Altitude(time)
	winter_azi[t] = Azimuth(time)
	time = time + 10 * Minute
 }
 print Date, " sunrise ", sunrise, " sunset ", sunset, " sunlight ", sunlight
 Date = "22-06-2017"
 call 'solar_params.dem'
 array summer_alt[150]; array summer_azi[150]
 time = -12 * Hour
 do for [t = 1 : 150] {
	summer_alt[t] = Altitude(time)
	summer_azi[t] = Azimuth(time)
	time = time + 10 * Minute
 }
 print Date, " sunrise ", sunrise, " sunset ", sunset, " sunlight ", sunlight
 Date = strftime("%d-%m-%Y",time(0) + Day*Longitude/360.)
 call 'solar_params.dem'
 array today_alt[150]; array today_azi[150]
 time = -12 * Hour
 do for [t = 1 : 150] {
	today_alt[t] = Altitude(time)
	today_azi[t] = Azimuth(time)
	dummy[t] = time
	time = time + 10 * Minute
 }
 print Date, " sunrise ", sunrise, " sunset ", sunset, " sunlight ", sunlight
 today = "Today ☉ \n".sunlight."\nof sunlight"
#
set angle degrees
set polar
set grid polar
unset xtics 
unset ytics
unset border
unset key
set size square
set rrange [90:0]
set ttics add ("N" 0, "E" 90, "S" 180, "W" -90) font ":Bold"
set rtics format "%.0f°"
set rtics 30
set rlabel "Altitude" offset -2 font ":Bold"
set border polar
set style data lines
# By convention North (azimuth 0°) is at the top with angles increasing clockwise
set theta top clockwise
plot dummy using (winter_azi[1ドル]):(winter_alt[1ドル]) lw 2 title " Winter Solstice" at beg left, \
 dummy using (summer_azi[1ドル]):(summer_alt[1ドル]) lw 2 title " Summer Solstice" at beg left, \
 dummy using (today_azi[1ドル]):(today_alt[1ドル]) lw 2 title today at end right

Click here for minimal script to generate this plot

set encoding save_encoding